All nervous systems contain large numbers of different neurotransmitters and neuromodulators. One goal of the proposed research is to test a model that suggests that many of these different substances are released by modulatory neurons to differently configure neural circuits, so that they can produce a number of different outputs. The stomatogastric system of decapod crustaceans will be used as a model preparation. Immunocytochemical and biochemical techniques will be used to identify additional neuropeptides present in the nervous system. Electrophysiological techniques will be used to study the effects of these peptides when exogenously applied to the stomatogastric ganglion. The synaptic contacts made by identified peptidergic modulatory neurons will be studied physiologically and anatomically. The motor patterns produced in vivo by the STG will be measured by electromyography, and compared to the motor patterns produced by the isolated stomatogastric nervous system. The data generated in these studies will provide basic understandings of the mechanisms underlying the generation of rhythmic motor patterns and their modulation by inputs from the central nervous system.